[Patent document 1] JP 2010-096743 A (US 2010/0073181 A1)
[Patent document 2] JP 2008-111809 A (U.S. Pat. No. 7,796,017 B2)
Conventionally, an example of an electrostatic type occupant detector, having a sub electrode in addition to a main electrode for the purpose of improvement in a signal to noise ratio (SN ratio) by increasing an electrode area, has been disclosed (e.g., see Patent document 1).
Further, an example of an electrostatic capacity occupant detection sensor to calculate an impedance between a main electrode and a sub electrode and perform discrimination based on an imaginary number part of the impedance between the main and sub electrodes, for the purpose of improvement in seat wet discrimination accuracy, has been disclosed (e.g., see Patent document 2).
However, in a seat, in addition to a seating face portion including a surface on which an occupant is seated, a non-seating face portion may also be wet or in a high humidity state. Note that the “non-seating face portion” means a part other than the seating face portion. More particularly, a part inside the seat, a lower part of the seat or the like correspond to the non-seating face portion. For example, “getting wet” is a state where water beyond an absorptive amount is spilt from the seating face portion. The seat may be in the high humidity state when an occupant is seated for a long time. Otherwise, the seat may be in the high humidity state in a rainy season or upon rise of fog.
When the technique disclosed in the above-described Patent document 1 is applied, it is merely detection of an electrostatic capacitance which occurs between (i) the main electrode and the sub electrode and (ii) the occupant (human body). Accordingly, in some cases, it is not possible to accurately perform occupant discrimination due to the influence of water which exists inside or in the lower part of the seat.
Further, when the technique disclosed in the above-described Patent document 2 is applied, it is merely detected whether or not both the main electrode and the sub electrode are wet in a selected predetermined mode (wet state detection mode). Accordingly, it is impossible still in the wet state detection mode to discriminate whether or not the seat is in a vacant seat state, an adult seated state (e.g. “AF05” indicating weight regular distribution) or a CRS (Child Restraint System) attached state. Accordingly, upon discrimination of occupant presence/absence, it is necessary to select another mode (occupant detection mode), i.e., to select another connection, to perform the discrimination.